This invention relates to a series-connected assembly of switching elements in a chopper which is located between a D.C. power source and a load and which controls D.C. power to be fed from the D.C. power source to the load.
In general, the series connection of switching elements in a chopper is performed in case a D.C. supply voltage can become higher than the breakdown voltage of one switching element (the collector-emitter breakdown voltage when the switching element is a transistor).
Heretofore, a device of this type has been as shown in FIG. 1. Referring to the figure, numeral 1 designates a D.C. power source. Numerals 2 and 3 designate transistors as switching elements, which are connected in series. Numerals 4 and 5 indicate diodes, which are also connected in series. The series-connected assembly of the transistors 2 and 3 and that of the diodes 4 and 5 are connected in a series inverse relationship so as to pass currents in directions reverse to each other, and both the ends of the resulting inverse series-connected assembly are connected to the corresponding ends of the D.C. power source 1. Numeral 6 indicates a load, both the ends of which are connected to the corresponding ends of the series assembly of the diodes 4 and 5. It is supposed that the load 6 is composed of a reactor and a resistor equivalently. Numerals 7 and 8 indicate overvoltage suppressors, which are connected in parallel with the transistors 2 and 3, respectively. Various overvoltage suppressors have been put into practical use, and an example thereof is shown in FIG. 2. In this figure, numeral 101 indicates a transistor, which corresponds to the transistor 2 or 3 in FIG. 1. Numeral 102 denotes a Zener diode as an overvoltage suppressor, which corresponds to the overvoltage suppressor 7 or 8 in FIG. 1. When the collector-emitter breakdown voltage of the transistor 101 is made at least the Zener voltage of the Zener diode 102 in advance no overvoltage is applied to the transistor 101. When, in the arrangement of FIG. 1, the voltage of the D.C. power source 1 is made less than double the Zener voltage in advance, any current does not flow steadily through the Zener diodes being the overvoltage suppressors 7 and 8. In the circuit of FIG. 1, the Zener voltage is usually selected such that (half of the voltage of the D.C. power source 1) &lt; (Zener voltage) .degree. (voltage of the D.C. power source 1).
Shown at numeral 9 is a control unit, which delivers "on" and "off" signals to the transistors 2 and 3 for the purpose of controlling D.C. power to be supplied from the D.C. power source 1 to the load 6. Numeral 10 represents a lead which is connected to the base of the tranndicates a current detector, which detects current flowing through the load 6. Various current detectors have been put into practical use, and one employing a Hall element is illustrated here. Since the current detector employing the Hall element is common, the explanation thereof is omitted. Among connection leads 15-18 between the control unit 9 and the current detector 14, the connection leads 15-17 are respectively connected to the zero, plus and minus terminals of a control power source included in the control unit 9, and the connection lead 18 is a lead which provides a voltage of a value proportional to the value of the current flowing through the load 6 (current detection signal) with respect to the connection lead 15.
FIG. 3 shows an example of the control unit 9. In the figure, numeral 103 indicates the control power source, and symbols .gradient., .chi. and .chi. denote the respective terminals of zero, plus and minus. Numeral 104 represents a current command device, which commands the current to flow through the load 6. The control of the current to flow through the load 6 is equivalent to the control of electric power to be fed to the load when the characteristics (impedance, counter electromotive force, etc.) of the load. Shown at numeral 105 is a comparator which has a hysteresis characteristic and which receives a current command signal Sr from the current command device 104 and the current detection signal Sd from the current detector 14 in FIG. 1 (the signal of the connection lead 18) so as to generate the "on" or "off" signals for the transistors in dependence upon the difference of the received signals.
FIG. 4 illustrates such a relationship, and its lower waveform depicts the output of the comparator 105. In the figure, a point A is the point at which a value obtained by subtracting the current detection signal Sd from the current command signal Sr has reached a positive hysteresis width and at which the signals of the transistors 2 and 3 change from the "off" signals to the "on" signals. When the transistors 2 and 3 have turned "on", the current which flows through the load 6 increases to reach a point B. The point B is the point at which the value obtained by subtracting the current detection signal Sd from the current command signal Sr has reached a negative hysteresis width and at which the signals of the transistors 2 and 3 change from the "on" signals to the "off" signals. When the transistors 2 and 3 have turned "off", the current which flows through the load 6 decreases to reach a point C. The point C is the same as the point A in the relationship between the current command signal Sr and the current detection signal Sd. Thenceforth, the transistors 2 and 3 are repeatedly turned "on" and "off", whereby the current to flow through the load 6 is controlled.
Referring to FIG. 3 again, numerals 106 and 107 indicate base power source-and-amplifiers, which amplify the transistor "on" or "off" signal from the comparator 105 and then deliver the amplified signals to the transistors 2 and 3. Since the base power source-and-amplifiers 106 and 107 are commonly used, the detailed description thereof is omitted. Connection leads 10-13 and 15-18 in FIG. 3 are the same as in FIG. 1.
Now, operations at the turn-on and -off of the transistors 2 and 3 in FIG. 1 will be described in detail with reference to FIG. 4.
First, when the transistors 2 and 3 are brought into their "on" states by the control unit 9, current from the D.C. power source 1 is applied to the load 6 through these transistors 2 and 3. When the current flowing through the load 6 has reached, for example, the point B in FIG. 4 in excess of a current value set by means of the current command device 104 (FIG. 3), the control unit 9 delivers the "off" signals to the transistors 2 and 3. When the transistors 2 and 3 have fallen into their "off" states, the current flowing through the load 6 is reduced while circulating via the diodes 5 and 4. Thus, when the current has reached, for example, the point C in FIG. 4, the control unit 9 delivers the "on" signals to the transistors 2 and 3. When the transistors 2 and 3 have fallen into the "on" states, the current from the D.C. power source 1 is applied to the load again, and the current value increases gradually. In this way, the current substantially equal to the current set by the current command device 104 flows through the load 6. This current flowing through the load zigzags as indicated by S.sub.1 in FIG. 4, on account of the hysteresis characteristic of the comparator 105 shown in FIG. 3.
The chopper arrangement shown in FIG. 1 can cause the current of the desired value to flow through the load 6 by operating as described above. However, the transistors 2 and 3 are seldom switched from "on" into "off" or vice versa at quite the same time. This leads to the disadvantage that a heavy loss of power is incurred by dispersion in the switching times of the transistors or by dispersion in the transfer times of the base power source-and-amplifiers in FIG. 3.
This disadvantage will be explained more concretely as to the case where the transistors 2 and 3 are switched from "off" into "on". By way of example, let it be supposed that the transistor 2 has turned "on" earlier. Then, since the Zener voltage of the overvoltage suppressor or Zener diode 8 connected in parallel with the transistor 3 is lower than the voltage of the D.C. power source 1, current flows along the D.C. power source 1, transistor 2, overvoltage suppressor 8, load 6 and D.C. power source. Since the overvoltage suppressor or Zener diode 8 allows the current to flow therethrough while holding the Zener voltage, the loss is considerably heavy. This condition continues till the turn-on of the transistor 3, at which the normal power is applied to the load 6. In this manner, when only one of the transistors 2 and 3 is in the "off" state during every switching operation of these transistors from "off" into "on" or vice versa, the Zener diode connected in parallel with the either transistor becomes conductive to incur the power loss as stated above.
As understood from the above, the prior-art chopper arrangement shown in FIG. 1 has the disadvantage that the loss of the overvoltage suppressor is affected by the discrepancy of the turn-on or turn-off times of the transistors 2 and 3 and that unless the dispersion is lessened, the loss becomes heavy.